Pulse code railway signalling systems typically are composed of various transceiver stations at opposite ends of track circuits which exchange pulse codes in a time sharing mode with a 2.8 second cycle time (some systems may use a 1.4 second cycle time, however). Electrical signals in the form of electrical pulses (pulse codes) are sent to these wayside stations by other, "downstream" wayside stations. Signals are then operated which display different aspects (red, green, yellow or combinations in accordance with the pulse codes). During the first 1.4 second half-cycle, a pulse code is sent by the transmitter portion of a first transceiver via the rails and is received by the receiving portion of a second transceiver. During the second 1.4 seconds of the cycle, information is received by the receiving portion of the first transceiver, thereby providing bidirectional signalling. The period of the coded information signals is thus limited to 1.4 seconds. Because of the poor electrical signal transmission characteristics of the rails (the medium over which the pulse coded signals propagate) which distorts the pulses, the duration of various pulses comprising the coded signals necessarily is long, on the order of 100 milliseconds. The long pulse duration limits the number of distinct codes which can be conveyed during the 1.4 second (half-cycle) transmission periods. The pulse codes must also be fail-safe/vital, such that if a failure occurs in transmission or reception, the railway signals display a safer aspect than the aspect associated with the transmitted pulse code.
At present, the number of vital codes which can be transmitted during any one 1.4 second time period is five. The number is limited by distortion and attenuation caused by the rails. This requires that each electrical pulse sent be of a minimum duration to ensure first that a readable portion of the signal is received and second that multiple pulses be separated in time such that they are received for decoding during one of five time windows.
To achieve the system reliability required for vital signal coding, it has been recent practice to limit the number of electrical pulses constituting vital codes to two. The time spacing between the two pulses is used to define the vital information.
It is a principal object of this invention to provide an improved railway signalling system, and particularly a system which can accommodate the transmission of an increased number of vital information codes during the industry-standard 1.4 second half-cycle time.
It is another object of this invention to provide an improved coding method which can be used in existing railway signalling systems with only a slight modification of existing logic to increase the number of vital information codes which can be transmitted during the industry standard 1.4 second half-cycle.